Solid‐State Enzymatic Hydrolysis of Mixed PET/Cotton Textiles**

Abstract

Waste polyester textiles trap copious amounts of useful polymers, which are not recycled due to separation challenges and partial structural degradation during use and thermo-mechanical recycling. Chemical recycling of polyethylene terephthalate (PET) through depolymerization can provide a feedstock of recycled monomers to make "as-new" polymers, and reduce the accumulation of plastic waste in landfills. Enzymes are highly specific, renewable, environmentally benign catalysts, with hydrolases available that are active on common PET textile fibers and on cotton. The enzymatic PET recycling methods in development, however, have thus far been limited to clean, high-quality PET feedstocks, and most such processes require an energy-intensive melt-amorphization step ahead of enzymatic depolymerization. Here we report that high-crystallinity PET in mixed PET/cotton textiles can be directly and selectively depolymerized to terephthalic acid (TPA) by using a commercial cutinase from Humicola insolens under moist-solid reaction conditions, affording up to 30 ± 2% yield of TPA. The process is readily combined with cotton depolymerisation through simultaneous application of cellulase enzymes (CTec2®), providing up to 83 ± 4% yield of glucose without any negative influence on the TPA yield. The herein presented selective and/or simultaneous enzymatic hydrolysis of PET/cotton textiles in solid reaction mixtures can expand the biocatalytic recycling processes of PET to less-valuable waste materials, and significantly increase its profitability through operating at very high solid-loading (40%), without the need for melt-amorphization.

Mechanoenzymatic reactions for the hydrolysis of PET

“…41 Similarly, recent reports of mechanoenzymatic PET degradation by HiC also employed a short RAging cycle of 5 min milling, 7 days aging. 17,45 A key difference here is our use of whole cell enzymes, and it was considered that more RAging cycles would be necessary to lyse the cells in situ.…”

“…17,45 In the HiC degradation of PET powder under 'moistsolid' conditions, short milling times were used followed by long periods of aging, leading to good yields of TPA. 17,45 Immobilised enzymes and non-immobilised puried enzymes in solution or lysates are normally used for mechanoenzymatic reactions, for example, immobilised CALB has been described together with RAging procedures. 46,47 In a further development, whole cell mechanoenzymatic strategies have been reported using transaminases under RAging conditions.…”

“…Recent studies have highlighted the mechanoenzymatic depolymerisation of PET using lyophilised HiC (Novozym ® 51032). 17,45 In the HiC degradation of PET powder under ‘moist-solid’ conditions, short milling times were used followed by long periods of aging, leading to good yields of TPA. 17,45…”

“…17,45 In the HiC degradation of PET powder under ‘moist-solid’ conditions, short milling times were used followed by long periods of aging, leading to good yields of TPA. 17,45…”

“…41,42 Recent studies have highlighted the mechanoenzymatic depolymerisation of PET using lyophilised HiC (Novozym ® 51032). 17,45 In the HiC degradation of PET powder under 'moistsolid' conditions, short milling times were used followed by long periods of aging, leading to good yields of TPA. 17,45 Immobilised enzymes and non-immobilised puried enzymes in solution or lysates are normally used for mechanoenzymatic reactions, for example, immobilised CALB has been described together with RAging procedures.…”